What salad is sort of unhelpfully explaining about the magic gems goes back to dimishing returns. I forget what the soft cap is but there is a value for magic or melee that over which everything you add is divided by something like 12. You may add 1000 rating but the effect on damage only goes up by 80ish. At that point it's better to invest in health than a tiny extra amount of damage. I'm assuming this point is close to one t7 gem on a high level x4 piece of gear based on salad saying you only need one magic gem.